Sulphuryl fluoride

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Sulphuryl fluoride has been developed as an effective fumigant for controlling dry wood termites. This gas has outstanding dispersion and penetrating qualities which permit it to infiltrate termite tunnels and crevices and destroy the insects. Sulphuryl fluoride does not escape through plastic sheets used in structural fumigation as rapidly as methyl bromide or other organic fumigants. Because this gas is odourless, chloropicrin, discharged separately, is recorernended as a warning agent.

PROPERTIES OF SULPHURYL FLUORIDE

Natural vapour pressure at different temperatures

10°C (50°F) 9 150 mm Hg
25°C (77°F) 13 442 mm Hg

Weights and volumes of liquid

1 lb at 4°C (39.2°F) has volume 338 ml
1 U.S. gal weighs 11.17 lb (5.069 kg)
1 Imp gal weighs 13.42 lb (6.087 kg)
1 kg has volume 745.1 ml
1 litre weighs 1.342 kg

Dosages and concentrations of gas in air (25°C and 760 mm pressure)

¹Ounces per 1000 cubic feet or milligrammes per litre

TOXICITY

Although highly toxic to humans acutely exposed, there have been few reports of accidental poisoning. This may be due to the fact that sulphuryl fluoride aerates very rapidly from fumigated buildings and also because, in its present usage, it is only applied by properly qualified operators. Its mammalian toxicity by inhalation, is about equal to that of methyl bromide.

Sulphuryl fluoride is generally very toxic to all postembryonic stages of insects (Kenaga, 1957b; Bond and Monro, 1961), but the eggs of many species are extremely resistant. It has been suggested that this resistance is largely due to the impenetrable nature of the eggshell layers to this chemical (Outram, 1967).

EFFECT ON MATERIALS

The effect of sulphuryl fluoride on materials found in houses and on plants and products has been summarized by Gray (1960) as follows.

"In laboratory and field tests, sulphuryl fluoride has shown no objectionable colour, odour, or corrosive reactions to photographic supplies, metals, paper, leather, rubbers, plastics, cloths, wallpapers or any other of a large number of articles fumigated.

"Sulphuryl fluoride has little or no effect on the germination of weed and crop seeds; however, it is in jurious to green plants, vegetables, fruits and tubers. Sulphuryl fluoride is sorbed less than methyl bromide in wheat, wood flour and many other materials."

Meikle and Stewart (1962) found that in fumigation with this compound the residual fluorides in foodstuffs are low except in certain proteinaceous foods which have a solvent system, such as fat in cheese and meat.

At the present time the manufacturers of a proprietary fumigant of sulphuryl fluoride (Dow Chemical Company, 1963) state specifically:

"Under no conditions should sulphuryl fluoride be used on raw agricultural food commodities, or on foods, feeds or medicinals destined for human or animal consumption. Do not use on living plants."

METHODS OF ANALYSIS

Determination of Vapours

Gross determination of concentrations of sulphuryl fluoride may be made with the thermal conductivity meters described in Chapter 4. fly using known concentrations of the gas, these instruments may be calibrated to read the gas concentration in terms of oz/1000 ft³. For instruments of known calibration for methyl bromide, A straight-line relationship exists for reading concentrations of sulphuryl fluoride. For example, a reading of 7 g/m³ is equivalent to 4 9 of sulphuryl fluoride/m3. The accuracy of thermal conductivity meters decreases with decreasing concentrations to the point where the readings become unreliable at concentrations of sulphuryl fluoride below 4 g (approximately 960 ppm)/m3. However, this is sufficiently accurate for normal commercial fumigation practices. Heuser (1963) described two analytical methods suitable for sulphuryl fluoride determinations under field conditions. These methods would also be useful for checks on the efficiency of the thermal conductivity analysers. For monitoring residual concentrations of this fumigant during the aeration of buildings, the manufacturers are careful to point out that the halide lamp described above under methyl bromide is not suitable for sulphuryl fluoride. It is recommended that a special device developed by them be used (Gray, 1960; Dow Chemical Company, 1963). The infra-red analyser described in Chapter 3 can be used for sensitive analysis of this fumigant. The Dow Chemical Company now recommends a modified portable SO2 analyser for excellent accuracy, fast measurements and low cost.

APPLICATION

Sulphuryl fluoride for termite control is most often applied to residences or other buildings, which are covered with gas-proof sheets. The fumigant is discharged directly from siphoned cylinders under its own vapour pressure. No auxiliary source of heat is required. The cylinders are placed on platformtype scales and the dosage read directly by change in weight. Metering devices are not safe and are not recommended. This fumigant will discharge through 0.3 cm (inside diameter) thick-walled polyethylene plastic tubing at the rate of 2 to 2.5 kg per minute.

For the purpose of monitoring gas concentrations so as to provide efficient and economical treatments to control termites in buildings, the manufacturers of sulphuryl fluoride supply special guide charts. These charts give factors for calculating the variables likely to be encountered during a fumigation and are best used in conjunction with the thermal conductivity analyser (Stewart, 1966).

PRECAUTIONS

Concentrations Toxic to Humans

The threshold limit for sulphuryl fluoride is 5 ppm for repeated eighthour exposures five days per week (ACGIH, 1981). The short-term exposure limit should not exceed 10 ppm (see Chapter 3, Table 7).

Respiratory Protection

In proper practice with this material there is no need for the operator to be exposed to the fumigant. In planning a fumigation, care should be taken to eliminate the possibility of anyone breathing any concentration. This is not only good general fumigation practice, but is also particularly important with sulphuryl fluoride because the standard respirator canister affords protection for a very short time, owing to the nature of the gas. According to instructions on the label issuer) by the manufacturer, a special canister designed for sulphuryl fluoride and other acid gases is required.

Protection from concentrations up to 32 g/m³ for 15 minutes can be obtained with the appropriate canister. For higher concentrations of sulphuryl fluoride, or for longer periods of time, air-supplied or selfcontained breathing apparatus should be used.

FIRST AID

The manufacturers of sulphuryl fluoride supply a booklet giving detailed recommendations for first aid, with suggestions to the physician.

The following condensed information on first aid is supplied on the manufacturer's label:

"Send for a doctor in case of accident. If a person should be overcome from breathing this gas, immediately place patient in fresh air, face downward, with head slightly below level of lungs. Keep warm. If breathing stops, give artificial respiration.

Note to physician: First symptoms expected are those of nausea, respiratory irritation and central nervous system depression; excitation may follow. Treat symptomatically. There is no known antidote."

Acrylonitrile

Owing to its low limits of flammability, acrylonitrile is never used alone as a fumigant but always in a mixture with another suitable material, which reduces the possibility of fire or explosion. A commonly used formulation was made up of acrylonitrile 34 percent and carbon tetrachloride 66 percent by volume. In practice, therefore, the effects of acrylonitrile fumigation were dependent on the action of the mixture.

Acrylonitrile itself has a comparatively high boiling point. When the mixture is used in atmospheric furnigations, it is necessary to hasten evaporation by pouring the liquid over a piece of jute (burlap) or similar cloth placed in an evaporating pan near the ceiling, and, if possible, in the air stream from a nearby circulating fan or blower.

The mixture of acrylonitrile and carbon tetrachloride has been found useful for the following purposes:

1. As a "spot" fumigant in mill, bakery and processing machinery.

2. For use in atmospheric chambers for fumigating tobacco (Tenhet, 1954; Childs and Overby, 1967), nutmeats (shelled nuts) and dates. It does not penetrate into certain closely packed materials as readily as methyl bromide and therefore is not usually recommended for use with flour and other milled products.

3. For the vacuum fumigation of tobacco (Tenhet, 1957).

4. For fumigation of buildings to control dry wood termites.

PROPERTIES OF ACRYLONITRILE
Alternative names: vinyl cyanide, cyanoethylene, propene nit rife

Natural vapour pressure at different temperatures

0°C (32°F) 33.0 mm Hg
10°C (50°F) 54.8 mm Hg
20°C (68°F) 87.5 mm Hg
25°C (77°F) 105 mm Hg
30°C (86°F) 140 mm Hg
40°C (104°F) 214 mm Hg

Weights and volumes of liquid

1 lb (avdp) at 20°C has volume 569
1 U.S. gal weighs 6.64 lb (3.014 kg)
1 Imp gal weighs 7.97 lb (3.615 kg)
1 ml 1 kg has volume 1 254.7 ml
1 litre weighs 0.797 kg

Dosages and concentrations of gas in air (25°C and 760 mm pressure)

¹Ounces per 1000 cubic feet or milligrammes per litre

TOXICITY

Acrylonitrile is highly toxic to humans when ingested, inhaled or absorbed through the skin. It exhibits many of the effects of hydrogen cyanide and is a severe skin and eye irritant. There are indications that acrylonitrile is associated with certain types of cancer in workers exposed over long periods of time ( Tierney et al, 1979) . A threshold limit value of 2 ppm was proposed in 1950 for adoption by the American Conference of Governmental Industrial Hygienists.

Acrylonitrile is very toxic to insects. It was found to be the most toxic of the more important fumigants used against several stored products insects (Bond and Monro, 1961; Lindgren et al, 1954; Harein and Soles, 1964; Rajendran, 1980). (See also Chapter 14, Table 16. ) The present use of acrylonitrile as a grain fumigant is under review in some countries because of suspected toxic effects to humans. Consequently, its future use may be limited or prohibited on this account.

EFFECT ON PLANT LIFE

Seeds

Acrylonitrile alone or in mixture with carbon tetrachloride is reported as not affecting the germination of a wide range of vegetable, cereal and flower seeds (Glass and Crosier, 1949; L indgren et al, 1954) . However, C.H. Richardson (1951) found them both detrimental to the germination of m a i z e .

Plants and Trees

Acrylonitrile is highly toxic to nursery stock and growing plants. It is not recommended as a plant fumigant either alone or in mixture.

EFFECT ON PLANT PRODUCTS

Fresh Fruit

Acrylonitrile seriously damages many fresh fruits (Claypool and Vines, 1956) .

Veqetables

Pradhan et al (1960) in India found that a 1: 1 mixture by volume of acrylonitrile and carbon tetrachloride could be employed to control the potato tuber moth, Phthorimaea opercullela (Zell.), in stored potatoes without in juring the tubers.

Cereals

Acrylonitrile-carbon tetrachloride mixtures have been recommended fur the control of insects in stored grain (Cotton and Young, 1943; Ruppel et al, 1960) .

RESIDUES IN FOODSTUFFS

There is little information available on the residues remaining in material streated with acrylonitrile . The fumigant sorbed by several commodities and desorption may require many days, depending on the type of commodity and the aeration conditions. Residual fumigant was found to desorb most rapidly from groundnuts and maize and slowest from wheat (Dumas and Bond, 1977). In admixture with carbon tetrachloride was found to disappear from spelled walnuts more rapidly than carbon tetrachloride (Berck, 1960). Residues of both fumigants were lower following vacuum fumigation for three hours than after treatment at atmospheric pressue for 18 and 48 hours .

METHODS OF ANALYSIS

A number of procedures using gas chromatography that will give rapid and precise analysis of acrylonitrile have been developed. Gawel (1979) described a headspace gas-chromatographic method for determination of concentrations down to 0.005 mg/kg, and Dumas and Bond (1977) gave a method for extraction of residual acrylonitrile from food materials.

Detector tubes described by Dumas and Monro (1966) may be used for field determinations at both fumigation and threshold limit concentrations. The presence of carbon tetrachloride in admixture has no effect on the readings for acrylonitrile.

APPLICATION

Owing to the high boiling points of the two components of the acrylonitrile-carbon tetrachloride mixture, special provision has to be made for rapid volatilization in atmospheric fumigation. Childs and Overby (1967) described the use of cotton rope wicks drawn through the bottom of a shallow steel pan. At the beginning of the treatment, liquid fumigant is poured into the pan and a fan blows air over the wicks for the time needed to complete volatilization.

Minor fumigants

In addition to the more important fumigants discussed above there are others with limited use, often for some specific application. The compounds listed in Chapter 2, fable 1 for reference purposes and which are not further discussed, are ethyl formate, methyl formate and parsdichlorobenzene.

Besides the fumigants given in Table 1, there are a few compounds that should be mentioned; these either show promise for future applications, or have a restricted use at present.

ACETALDEHYDE

Acetaldehyde is a naturally occuring volatile compound that is toxic to insects as well as certain fungi, bacteria and yeasts. Tests on fruit and vegetables have shown that it can be used to control some insects, such as aphids and thrips, without undue injury to the fresh produce (Aharoni et al, 1979, 1980). Concentrations up to 3 percent for 30 minutes have been applied to apples as a fungicidal treatment without causing injury to the fruit (Stadelbacher and Prasad, 1974).

The boiling point of acetaldebyde is 21°C, it has a characteristic pungent odour and is flammable at concentrations of 4 percent or greater. Acetaldehyde is less toxic to mammals than other commercial fumigants, such as ethylene dibromide. It has a narcotic action on the nervous system and causes irritation to the eyes and mucous membranes. Large doses may cause death by respiratory paralysis. This compound is suspected of having carinogenic activity in humans (Obe et al, 1979); however, it is not likely to be hazardous as a residue on fresh produce because it does not accumulate in tissues (Fidler, 1968).

Plant tissues may absorb acetaldehyde but it is metabolized to acetic acid, ethanol and carbon dioxide. Acetaldehyde occurs naturally in fruit and vegetables, it is used as a flavouring agent and has been registered as a food additive in the United States. This compound and other naturally occurring volatile compounds, such as ethyl formate, may have some potential as future replacements for fumigants that leave harmful residues.

AZ0BENZENE

This compound (boiling point 293°C) is a crystalline solid which has proved useful in the past for the control of mites in greenhouses. It acts as a vapour formed by heating the pure crystals by means of steam pipes, hot plates or lamps; it is also evolved by igniting a powder in a pressure fumigator. Many glasshouse plants and blooms are tolerant, but there has been some discoloration of red flowers (Pritchard, 1949). Flowers of the African voilet (Saintpaulia) are reported to be susceptible to damage (Brown, 1951).

CHL0R0F0RM

Chloroform (boiling point 61°C) is not highly toxic to insects but it has shown considerable promise as a constituent of liquid-type fumigants, in which it serves as a carrier for other more toxic ingredients, such as ethylene dibromide and carbon disulphide. Chloroform is nonflammable when mixed in air in any proportion. However, it is listed as being a compound which is "suspect of inducing cancer" by the American Conference of Governmental Industrial Hygienists (ACGIH, 1981).

DICHLORONITROETHANE

This fumigant (boiling point 124°C) was introduced some years ago and appeared useful for treating grain, other stored products and soil. It gives warning by odour and irritation of the eyes. It may be strongly corrosive to metals in moist atmospheres.

ETHYLENE CHLOROBROMIDE

Ethylene chlorobromide is effective against the oriental fruit fly, but it is not as toxic as the closely allied ethylene dibromide (Balock and Lindgren, 1951). It has a lower boiling point than EDB. The effect of this fumigant on fruit and plant material is similar to that of EDB. It has not come into general use, although it has been adequate in many fields of application for which EDB is also suited (Benschoter, 1960, 1963; Wolfenbarger, 1962; Sinclair et al, 1964; Richardson and Roth, 1966).

METHYL ALLYL CHLORIDE

This gas, also called methallyl chloride, has been suggested as a grain fumigant and has been used experimentally in mixtures with other fumigants. In laboratory tests it has ranked above carbon disulphide and ethylene dichloride against eight species of stored-product insects (Lindgren et al, 1954) and it is considerably more toxic than carbon tetrachloride, although less toxic than ethylene dibromide. Methallyl chloride has a boiling point of 72°C and is nonflammable in concentrations toxic to insects. In field trials in Russia it has shown promise as a fumigant for cereal grains and pulses and is significantly less expensive to apply than chloropicrin and methyl bromide (Cherkovskaya, 1963, 1966). Methallyl chloride also shows promise for fumigation of individual sacks of grain (Taylor, 1975).

METHYLENE CHLORIDE

Methylene chloride has been found useful as an ingredient of fumigant mixtures (see Chapter 7). It is nonflammable and has a boiling point of 40.2°C. In itself it is toxic to insects, but ranks low in toxicity in comparison with other commonly used materials (Back and Cotton, 1935).

NICOTINE

Nicotine (boiling point 247°C), vaporized in different ways, was formerly used extensively as a glasshouse fumigant. The most convenient method of volatilizing it is by the use of pressure cans; these are ignited to send off dense clouds of smoke containing the vapours of nicotine. In a glasshouse, it was found that the nicotine concentrations fell off rapidly and little or none remained one hour after discharge (Richardson et al, 1943a). When the nicotine was evaporated slowly, the concentration was maintained only as long as evaporation continued and the introduction of fumigant was equal to the loss through leakage(Blackish, 1953). Delicate flowers and tender shoots may be injured by nicotine applied in any way.

Nicotine fumigation in glasshouses has been largely replaced by the aerosol technique described in Chaper 12. Nevertheless, it is still used under certain conditions, for example when convenience in application is important.

PROPYLENE DICHLORIDE

In empty containers, this compound (boiling point 96.4°C) compares favourably with many other fumigants in toxicity to insects. It has been tried as a grain fumigant, usually in mixture with carbon tetrachloride, but so far has not been widely adopted (Cotton, 1963).

SULPHUR DIOXIDE

Sulphur dioxide (SO2) should be mentioned if only to point out its disadvantages. It is the oldest fumigant known to man (Cotton, 1963), having been used from time immemorial by burning sulphur.

Nowadays it is sometimes used as an ingredient of liquid grain fumigants. Intrinsically it is quite toxic to insects. It also serves as a warning gas in these mixtures on account of its intensely irritating properties to humans. It is rapidly sorbed by any material undergoing treatment; it has a deleterious effect on grain and flour and is highly corrosive to metals (Cotton, 1963).

METHYL CHLOROFORM (1,1,1 - TRICHLOROETHAME)

This compound has been tested as a possible substitute for carbon tetrechloride; it is slightly more toxic to insects than the latter but less toxic than ethylene dichloride. Its boiling point (74°C) and volatility are very similar to those of carbon tetrachloride. The vapour density of methyl chloroform is sufficiently greater than air that effective penetration into grain masses can be expected. When used as a 1 : 1 mixture with ethylene dichloride, an enhanced toxic effect was noted and the two compounds were found to interact physically to affect the distribution of each other in a grain bulk (UK, 1978).

Tests have indicated that residues of methyl chloroform should not be a toxic hazard in fumigated grain; no adverse effects were noted on bread made from fumigated wheat and virtually no residue could be detected in the loaves (UK, 1978).

Methyl chloroform is less toxic to humans than several other halogenated hydrocarbons of comparable function, including carbon tetrachloride. Animal and human toxicological data indicate that this compound should have little potential for producing permanent organic injury in humans, provided anaesthetic concentrations, sufficient to depress the respiratory centre, are not exceeded (Steward et al, 1969). The threshold limit value is 350 ppm (ACGIH, 1981). For the detection of low concentrations of methyl chloroform, the halide leak detector and glass detector tubes may be used.

CARBON DIOXIDE

Carbon dioxide (CO2) is an ingredient of our normal atmosphere and is not to be considered as a poisonous fumigant in the ordinary sense. Nevertheless, it has an important role in insect control (see Chapter 11).

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